Recent studies of Lomonosov Ridge sediments provided evidence for an Eocene lacustrine stage in the Arctic Basin and concluded at its merging into global oceanic circulation some 17.5 Ma ago, with a ∼26 Ma gap in-between. Here, Re-Os data are used to document these events. High Os-concentrations in the lacustrine unit and overlying transitional layer point to low sedimentary O2-conditions. Highly radiogenic Os-isotope values mark this layer suggesting massive input of old continental material. Above, Os concentrations decrease to typical oxic-sediment values, whereas 187Os/188Os-ratios follow the trend observed in other oceans. However, Re-Os isochrons from layers assigned to the upper and lower limit of the 26 Ma-gap yield identical ages, suggesting a short transitional event marking the opening of Fram Strait and a ca. 38 Ma for the inception of marine conditions. If confirmed, this new chronology would modify views about the tectonic evolution of Lomonosov Ridge.